The present invention relates to a method of manufacturing a high-power semiconductor laser device with long-term reliability.
A semiconductor laser has been used for apparatuses in various fields such as information communications, printings, processing, medical applications, or the like. It is necessary to improve power and reliability of the semiconductor laser as a light source, so as to improve the performance of these apparatuses.
In general, the semiconductor laser has a structure in which an active layer is sandwiched between a p-type cladding layer and an n-type cladding layer. Then, a substrate having the layers laminated thereon is cleaved and laser light is generated by applying a current to the active layer using the cleavage plane as a resonator plane. Then, one of two cleavage planes serving as resonator planes, becomes a light outputting part. Further, the two cleavage planes are coated with a dielectric film for controlling the reflectance or suppressing deterioration with time caused by chemical reaction on a cleavage surface.
When cleaving is carried out in general air atmosphere, a natural oxide film is formed on the cleavage surface. Taking GaAs compound as an example, high-density surface levels, which is mainly caused by oxygen binding of Ga and As, are present in the natural oxide film on the cleavage plane. Then, emitted light is absorbed by the natural oxide film as a non-radiative recombination center. Due to the light absorption, heat is generated in the vicinity of the cleavage plane, and a forbidden bandwidth of the active region is decreased, resulting in further increasing light absorption. Consequently, the cleavage plane is melted away, causing deterioration of the laser output considerably. Therefore, to achieve a high-power semiconductor laser with high reliability, it is necessary to preclude the formation of a natural oxide film formed on the cleavage plane, particularly.
Conventionally, to prevent the natural oxide film from being formed, the following processes are accomplished. That is, after cleaving is carried out in high vacuum, a protection layer is formed without exposing the cleavage plane to the air before forming a natural oxide film is formed, or after cleaving is carried out in an atmospheric air, the natural oxide film formed on the cleavage plane is removed by an electron beam heating, a laser irradiation, or plasma exposure using an inert gas so as to form a protection film. In addition, another method is also accomplished. That is, after placing the cleavage plane into a vacuum apparatus, the cleavage plane is exposed to a halogen gas at 400xc2x0 C. or higher. Then, an oxide layer is removed by thermochemical reaction, and a compound semiconductor layer and the like is formed thereon.
However, the above mentioned cleavage operation in high vacuum is required for extremely high vacuum level depending on the process time, resulting in requiring high cost or strict control of apparatuses.
Further, according to the method of forming a protection film by removing a natural oxide film by means of an electron beam heating, a laser irradiation, or plasma exposure using an inert gas, the natural oxide film or surface contaminants is removed by a physical method, mainly. Therefore, there is a concern that defects are introduced in a surface layer in addition to the removal of these. Using the above methods, in particular, oxygen binding of Ga and As can be removed, however, the introduced defects function as a recombination center. Consequently, it is necessary to perform precise control of processing conditions or the like for an improvement of these methods.
Further, according to the method of thermochemical reaction with a halogen gas, since it is necessary to heat the halogen gas to 400xc2x0 C. or higher, an electrode cannot be formed before the cleavage operation. Instead, an electrode is formed after forming a protection film for the resonator plane which is formed by cleaving. Consequently, there exists a problem in that processes become inconvenient and complicated.
The invention is proposed to solve the above problems. According to the invention, a natural oxide film formed on a cleavage plane is removed and also a protection film is formed by using a catalytic Chemical Vapor Deposition (CVD) apparatus.
Namely, the invention provides a method of manufacturing a semiconductor laser comprising the steps of:
laminating a semiconductor thin film comprising a well layer on a semiconductor substrate;
cleaving the semiconductor substrate and the semiconductor thin film;
exposing a cleavage plane of the semiconductor substrate and semiconductor thin film obtained by cleaving to an atmosphere produced by decomposition of a gas containing N atoms, under the presence of heated catalytic substances, thereby removing the surface layer of the cleavage plane and forming a nitride layer on the surface; and
subsequently forming a dielectric film on the cleavage plane.
According to the invention, even if the resonator plane of the semiconductor laser is formed by cleaving in the air, a surface layer made of a natural oxide film formed on the cleavage plane is exposed in a vacuum apparatus to a gas containing N-atoms, which are changed into radical in the catalytic CVD apparatus. By doing this, etching removal can be carried out at a low substrate temperature with extremely low level of damage of the semiconductor thin film, and at the same time a nitride layer, which has excellent chemical stability, can be formed. As the gas containing N-atoms, ammonia (NH3), hydrazine (NH2NH2), or the like can be used. Since the nitride layer has a wide band gap and terminates and decreases defects, it is very preferable material in view of junction between a semiconductor and a dielectric film. In general, when GaAs is used in III-V group semiconductor laser, however, a GaN layer is formed therein.
Subsequently, by forming a dielectric film on the cleavage plane, the dielectric film is formed on the face from which the natural oxide film is removed. Because of this, it is possible to prevent temperature from increasing due to light absorption and to prevent the cleavage plane from melting when emitting laser light. Then, since the nitride layer formed on the cleavage plane, from which the natural oxide film is removed, has excellent chemical stability, reoxidation will not occur even if the cleavage plane is exposed to the air. Therefore, between the step of exposing the cleavage plane to the atmosphere produced by decomposition of a gas containing N-atoms using the catalytic CVD apparatus and the step of forming the dielectric film, it is allowed to expose the semiconductor substrate to the air.
Further, in comparison with the case where plasma process such as sputtering is used for forming the dielectric film, the method in which after eliminating the natural oxide film and forming the nitride film by the catalytic CVD apparatus then forming the silicon nitride film by means of the catalytic CVD apparatus is preferable because plasma damage caused by ion impacts on the cleavage plane can be eliminated. That is, after removing the natural oxide film and forming the nitride film by means of the catalytic CVD apparatus, the silicon nitride film is subsequently formed by using the same catalytic CVD apparatus. The silicon nitride film is formed by exposing the cleavage plane to an atmosphere produced by decomposition of a gas containing N and Si, or a gas containing N and a gas containing Si, under the presence of heated catalytic substances.
In the invention, it is preferable that a well layer of the semiconductor laser manufactured by the above steps, is made of a composition of any elements selected from In, Al, Ga, P and As. These elements will form a chemically stable nitride film.